L36: Free Radicals, Antioxidants

Question 1

Describe the reaction sequence converting oxygen to water and identify the most reactive intermediate

Answer 1

1. Singlet oxygen
2. Superoxide anion (o2-): generated by adding 1 electron to oxygen
3. Hydrogen peroxide (h2o2): 2 electrons
4. Hydroxyl radical (Ho-): 3 electrons
5. Water generated by adding 4 electrons to oxygen
   • ~ 1% of oxygen consumed by ETC is converted into ROS

Question 2

Name five sources of ROS

Answer 2

1. ) ETC: main source at quinone linked to complex will generated superoxide anion
2. ) NADPH/Cytochrome P450 oxidase: generates superoxide anion
3. ) Cytochrome P450 oxidase as part of MEOS when handing ethanol: large amounts of H2o2 produced with hydroxyethylradical

Question 3

Describe how chronic alcohol consumption affects radical production in the liver

Answer 3

• MEOS P450 cytochrome system generates larges amount of hydrogen peroxide and hydroxyethyl
• Hydrogen peroxide damages membranes and kills cells
• DNA is damaged by hydroxyethyl creating mutations and predisposing one to cancer
• Xanthine oxidase: purine degradation, generates hydrogen peroxide
• Free copper and iron can donate electron to molecular oxygen or hydrogen peroxide, generating superoxide anion and hydroxide radical respectively

Question 4

Name three enzymes and reaction sequences that detoxify ROS

Answer 4

• 1.) superoxide dismutase: acts on superoxide to produce hydrogen peroxide and oxygen
• 2.) catalase: acts on peroxide to form water and oxygen
• 3.) glutathione peroxidase: acts on hydrogen peroxide to produce water and GSSG

Question 5

Name thee vitamins that protect from ROS

Answer 5

• Vitamin C (ascorbic acid)
• Vitamin E (tocopherols)
• Vitamin A (retinol)

Question 6

Explain how glutathione protects from oxidative stress

Answer 6

• glutathione = electron donor, 2 GSH produce 2 GSSGs when giving electrons
• GSSG regenerated by glutathione reductase via NADPH

Question 7

Identify three metabolic antioxidants (in addition to GSH)

Answer 7

• Polyphenols (flavonoids) founds in plant extracts
• Uric acid (antioxidant in serum)
• Bilirubin

Question 8

Describe how antioxidants work together to prevent damage to membranes

Answer 8

• Antioxidants donate electrons to free radicals with ultimate source originating from NADPH

Question 9

Explain how RBCs prevent/repair oxidative damage to membranes and hemoglobin

Answer 9

• RBCs have high oxygen saturation that leads to large production of ROS
• They employ five reactions to combat this: glutathione peroxidase, superoxide dismutase, catalase, (GSH, vit C, Vit E), methemoglobin reductase replaces electrons to iron in heme (turning it from Fe3+ to Fe2+ when electron stolen from it)

Question 10

Describe the reaction sequence leading to lipid peroxidation

Answer 10

• PUFA is acted on by hydroxyl radical to generate lipid radical
• Lipid radical react with oxygen to form lipid peroxides
• Lipid peroxides are unstable and abstract electrons from other PUFA double bonds and propagation occurs

Question 11

Explain reperfusion injury

Answer 11

• Sudden restoration of oxygen supply to hypoxic tissues produces a burst of ROS

Question 12

Explain why iron overload generates oxidative stress

Answer 12

• Iron is typically bound to ferritin and transferring in body. Overloading leads to free iron
• Excessive iron concentrations generate oxidative stress that leads to destruction of liver, pancreas and heart muscle cells
• Hereditary hemochromatosis is a common iron storage disease that predispose pts to accumulation of iron